Process and apparatus for producing a single crystal of semiconductor material

ABSTRACT

A process for producing a single crystal of semiconductor material, in which fractions of a melt, are kept in liquid form by a pulling coil, solidify on a seed crystal to form the growing single crystal, and granules are melted in order to maintain the growth of the single crystal. The melting granules are passed to the melt after a delay. There is also an apparatus which is suitable for carrying out the process and has a device which delays mixing of the molten granules and of the melt.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for producing a singlecrystal of semiconductor material by means of a method which differsfrom the known zone pulling (Fz process) substantially becausepolycrystalline granules, instead of a polycrystalline stock ingot,supply the material for the growth of the single crystal. The presentinvention also relates to an apparatus which is suitable for theproduction of the single crystal.

[0003] 2. The Prior Art

[0004] A process of the same general nature is already known from DE19538020 A1. The granules are melted in a vessel and fed to a melt whichis located on the growing single crystal. The growth of the singlecrystal is maintained by an equilibrium between molten granules fed tothe melt and solidifying fractions of the melt.

SUMMARY OF THE INVENTION

[0005] It is an object of the present invention to make it possible toproduce dislocation-free single crystals, in particular with diametersof 200 mm and above.

[0006] The above object is achieved by the present invention whichprovides a process for producing a single crystal of semiconductormaterial, in which fractions of a melt, which is kept in liquid form bya pulling coil, solidify on a seed crystal to form the growing singlecrystal, and granules are melted in order to maintain the growth of thesingle crystal, wherein the melting granules are passed to the meltafter a delay.

[0007] The present invention also provides an apparatus for producing asingle crystal, comprising a vessel which is arranged above the growingsingle crystal and a conveyor device for feeding granules into thevessel, and a melting coil for melting the granules, and a pulling coilfor maintaining a melt on the growing single crystal, the meltinggranules passing through openings in the vessel and the pulling coil tothe melt, so as to form a melt neck, and solidifying fractions of themelt maintaining the growth of the single crystal, wherein the vesselhas a device which delays mixing of the molten granules with the melt.

[0008] The process of the invention makes it possible to produce singlecrystals with the characteristics of zone-pulled material at costs whichare well below the costs of Fz material. The polycrystalline granuleswhich supply the raw material for the crystal growth are significantlyless expensive than the polycrystalline stock ingots required for the Fzprocess. In addition, polycrystalline stock ingots are rarely availablein a quality and size which makes it possible to produce single crystalswith diameters of 200 mm and above. Yet even if this were possible, theprocess for pulling single crystals with such diameters can only becontrolled with difficulty. This is on account of the masses which haveto be simultaneously melted and crystallized. The consequence is lowyields of dislocation-free single crystals, which are not economicallycompetitive.

[0009] Although the process described in the abovementioned DE 19538020A1 avoids the problems presented by the production and use ofpolycrystalline stock ingots, it is unsuitable for the production ofdislocation-free single crystals. This is because of the particles whichoriginate from the granules. These particles can all too easily reachthe interface between the melt and the growing single crystal and endthe dislocation-free growth of the single crystal.

[0010] This situation is where the present invention achieves theseunexpected results by providing that the supply of the granules to themelt be delayed in its movement to the melt. As a result and as far aspossible granules can only reach the melt when they have been completelymelted. For this purpose, measures are taken to extend the distance overwhich the melting granules have to travel in order to reach the meltand/or to provide a barrier to these granules which have not yetcompletely melted. The melting granules preferably have to cover adistance of at least 25 mm, particularly preferably 50 mm, beforereaching the melt. Furthermore, the invention is distinguished by thefact that measures which effectively avoid uncontrolled inclusion ofoxygen in the single crystal are also provided. On the other hand,controlled amounts of oxygen can be fed to the melt via the growingsingle crystal, for example by positioning a ring of SiO₂ on the melt. Asuitable ring is described, for example, in U.S. Pat. No. 5,089,082.

[0011] High-frequency coils are in each case used to melt the granulesand to pull the single crystal. It is particularly advantageous if thepulling coil and the melting coil are inductively decoupled. This meansthat the energy provided by the pulling coil is used to control thegrowth of the single crystal but not to melt the granules. Decoupling ofthis nature can be achieved simply by leaving sufficient distancebetween the pulling coil and the base of the vessel to which thegranules are fed.

[0012] At the start of the process, a melt is produced on a seed crystalin a similar manner to that which is also customary in the Fz process.The volume of the melt, which initially only comprises a molten drop, isincreased as a result of the melting of the semiconductor material. Inparallel, fractions of the melt are made to solidify, so as to form agrowing single crystal, by slowly lowering the seed crystal withrotation. In a first phase, the single crystal is allowed to grow into acone. Later, the diameter of the single crystal is kept constant, withthe result that most of the single crystal acquires a cylindricalappearance. The semiconductor material which is required for theproduction of single crystals with diameters of 200 mm and above, inparticular during the pulling of the cylindrical section, is suppliedsubstantially by polycrystalline granules which are melted with the aidof the melting coil. The melting granules are fed to the melt with adelay. To keep particles away from the growing single crystal, it ispreferably ensured that the space around the vessel is separated in adustproof manner from the space around the growing single crystal. Inaddition to structural measures which promote such separation, it isdesirable for a gas stream, consisting, for example, of inert gas, suchas argon, to be fed from the bottom upward through the pulling coilduring the production of the single crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Other objects and features of the present invention will becomeapparent from the following detailed description considered inconnection with the accompanying drawings which disclose severalembodiments of the present invention. It should be understood, however,that the drawings are designed for the purpose of illustration only andnot as a definition of the limits of the invention.

[0014] The invention is described in more detail below on the basis offigures. Identical features are provided with identical referencenumerals.

[0015] FIGS. 1 to 4 show preferred embodiments of the apparatusaccording to the invention.

[0016]FIG. 5 shows a plan view of a melting coil which is particularlysuitable for use in an arrangement as shown in FIG. 4. In the furtherexplanations of the invention, silicon is mentioned as a particularlypreferred semiconductor material.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0017] In the apparatus shown in FIG. 1, there is a pot-like vessel 1,which can rotate and can be displaced in the axial direction, positionedabove a pulling coil 2. The vessel consists of SiO₂, for example quartz,and, like the pulling coil, has a circular opening 3 in the center. Itsinterior is divided into a plurality of, preferably at least three,regions, which form a system of passages, by concentric quartz walls 4.The individual regions are connected to one another by openings 6 insuch a way that the distance from the outer region to the centralopening 3 is as long as possible and, for example, is in meanderingform. In the regions there are individual or a plurality ofparallel-connected turns of a high-frequency coil which is used to meltthe granules and therefore serves as melting coil 5. In the outerregion, to which the granules 11 are fed, the coil turns are coveredwith covers 12 made from quartz in order to avoid contact between thegranules and the metallic surface of the melting coil. In the outerregion, the quartz walls 4 are designed in such a way that the granules11 supplied via a conveying device 10 cannot be scattered into the innerregions.

[0018] An ingot 7 of silicon, at which the molten silicon can run downthrough the inner hole in the pulling coil 2, to form a melt neck 18, tothe melt 8 on top of the growing single crystal 9, projects into thecentral opening 3 in the vessel. The ingot can rotate and can bedisplaced in both the radial and axial directions. The axis of rotationof the vessel 1 is tilted through a small angle α, thus ensuring thatthe ingot is always wetted at the same place relative to the pullingcoil 2. Radial displacement of the pulling coil makes it possible tocontrol the way in which molten material runs out of a pool of melt 17in the vessel 1 to the melt 8.

[0019] To prevent dust particles from being able to reach the melt 8,the space in which the single crystal is pulled should as far aspossible be separated in a dustproof manner from the space in which thevessel is located. It is therefore preferable for the sickle-shaped gapbetween the ingot 7 and the edge of the central opening 3 in the vesselto be as narrow as possible, and for a gas stream to be directed upwardthrough the gap, making it difficult for dust to penetrate into thepulling space.

[0020] Production of a single crystal begins by first of all melting asmall quantity of silicon in the vessel 1 and keeping it in liquid form.In this phase, the ingot 7 is not yet in contact with the pool of melt17 which has been produced. Then, the ingot is moved downward throughthe central opening 3 in the vessel and the inner hole in the pullingcoil. The seed pulling is commenced in a known way as a result of amolten droplet being produced on the lower tip of the ingot with the aidof the pulling coil 2 and a seed crystal being attached to this moltendroplet. At this time, the ingot still has the function of the stockingot used in the Fz process. First of all, as a result of the ingotbeing melted further and as a result of the lowering of the seed crystalcommencing, a starting cone in single crystal form with a melt ofsufficient volume resting on it is produced. Then, the ingot, togetherwith the pulling coil, is displaced synchronously in such a way that thematerial which has been melted in the vessel comes into contact with theingot and as a result liquid silicon can move along the ingot to themelt neck 18 and, from there, to the melt 8 on the growing singlecrystal 9. As the process continues, granules 11 are fed to the vesselaccording to demand and are melted. The growth of the single crystal isnow substantially maintained by molten granules.

[0021] The extent of the axial displacement of the vessel 1 relative tothe melting coil 5 regulates the extent to which the HF field of thiscoil is introduced into the molten granules. The melting characteristicsof the granules can be influenced in this way and also by the choice ofthe HF power. Displacement of the vessel relative to the pulling coilmay also be advantageous for the control characteristics. If thedistance from the pulling coil becomes great, energy is no longerintroduced into the pool of molten granules from below, and siliconfreezes at the bottom of the vessel. If the shape of the pulling coil isadditionally modified in such a way that an upward bulge is formedintegrally on the wetting side where the pulling coil adjoins the ingotwhich has been wetted with liquid silicon, at this location the locallyhigher introduction of energy means that no silicon freezes on the baseof the vessel. Therefore, the molten granules can continue to run downto the melt without defects, while at the same time the direct contactsurface between the molten granules and the base of the vesselconsisting of SiO₂ is minimized by the layer of frozen silicon. Thismakes it possible to considerably reduce the introduction of oxygen intothe melt and the formation of SiO.

[0022] In the embodiment shown in FIG. 2, the vessel 1 comprises a plateof silicon which in the center has a tubular opening 3 which is createdby a section of pipe 13 which is drawn downward. The plate is mountedrotatably, preferably on three wheels 14 which support the plate at theedge and also serve as a rotary drive. The plate 1 and the integrallymolded section of pipe 13 are protected against direct introduction ofthe HF field of the pulling coil 2 from below and from the side by acooling device 15. Device 15 can be for example a water-cooled metalplate, so that melting of the lower side of the plate 1 and of the outerside of the section of pipe by the pulling coil is prevented. Moreover,the metal plate acts as a heat sink which dissipates the heat generatedby the melting coil 5 in the plate. The melting coil is arranged abovethe plate. The central opening 3 in the plate and the inner side of theintegrally molded section of pipe 13 are heated by an additional energysource, for example a radiation heating means, which is illustrated as alens 16 for the purposes of simplification, in order to prevent freezingof the molten granules flowing to the melt and of the melt neck 18 whichforms. The thermal gradient which builds up in the plate and theintegrally formed section of pipe ensures that a stable pool of melt 17is formed on the top side of the plate and the inner side of the sectionof pipe remains in liquid form, while the base of the plate and theouter side of the integrally formed section of pipe remain in solidform. The section of pipe 13 is completely closed off at the bottom byliquid silicon of the melt neck 18. Concentric quartz rings 4, which, asin the embodiment shown in FIG. 1, define regions which are connected toone another by openings 6 in such a way that a meandering path isformed, which the molten granules have to overcome before they can reachthe melt 8, project into the pool of melt 17 which is formed by partialmelting of the upper side of the plate and by melting of the granules.The feed device 10 and the covers 12 have the same functions as in theembodiment shown in FIG. 1. In addition, the innermost cover, by meansof a suitable structural design, is now responsible for preventingsupplied granules from passing directly into the inner region of theplate.

[0023] The embodiment shown in FIG. 2 has the advantage that the surfacearea of contact with quartz and therefore the introduction of oxygeninto the melt 8 is reduced further, and that the melting of the granules11 and the pulling of the single crystal are completelyelectromagnetically decoupled. As a result, the pulling coil 2 can beoptimized purely with a view to the pulling operation. Control alsobecomes more stable. Furthermore, the inner molten surface of the meltneck 18 at the end of the section of pipe 13 acts as a barrier toindividual granules which have not yet completely melted, since theyfloat on the surface until they have melted. It is virtually impossiblefor such particles to reach the growth front of the single crystal andcause dislocations in the crystal lattice. A further advantage is thatthe space holding the growing single crystal 9 can be very successfullysealed in a dustproof manner from the space holding the plate 1, sincethe two spaces are only connected by a narrow annular gap between themetal plate 15 and the plate 1. The dustproof separation of the spacescan be improved even further by a protective shield 19.

[0024] The production of a single crystal begins by first of all meltinga closure at the lower end of the section of pipe and by a seed crystalbeing fitted and pulled into a cone in the manner which has already beendescribed. The closure used may be a piece of silicon which has beeninserted into the section or pipe or the melt neck which solidifiedafter the pulling of a previously produced single crystal. In thisrespect, the closure takes over the function of the ingot 7 shown inFIG. 1. At the same time or subsequently, the upper sides of the plate 1and the closure of the tubular central opening are melted with the aidof the melting coil 5 and the radiation heating means 16, and furthermolten material is fed to the growing single crystal. Then, as thedemand for molten material increases, additional granules are supplied,so that a stable pool of melt is formed on the upper side of the plate,from which there is a continuous, controllable flow of molten materialto the melt on the growing single crystal.

[0025] In the embodiment shown in FIG. 3, which is similar to theapparatus shown in FIG. 2, quartz walls which are in contact with thepool of melt are completely dispensed with, so that there is no oxygendoping of the single crystal or formation of SiO. Instead, the meltingcoil 5, in the region above the edge of the tubular opening, is designedin such a way that at that location an increase in height 20 is producedon the surface of the plate 1, forming a barrier. If the melting coil ismoved closer to the pool of melt or the HF power is increased, moltenmaterial is displaced by the repelling electromagnetic force and flowsover the barrier into the tubular opening 3. If the barrier issufficiently high, granules which have not yet completely melted areunable to overcome the barrier, on account of the force of gravity.Therefore, the barrier acts as a filter which blocks solid semiconductormaterial. Of course, the melting coil may be designed in such a way thata plurality of barriers in series are formed on the plate.

[0026] A single crystal is produced in a similar manner to the procedurewhich has already been described in connection with the embodiment shownin FIG. 2.

[0027] In the embodiment shown in FIG. 4, the concentric quartz walls ofthe apparatus shown in FIG. 2 have been replaced by solid webs 21 ofsilicon which project from the surface of the plate 1. The individualturns of the melting coil 5 are pulled relatively far apart on the innerside, so that between the turns the plate is not melted and webs remainin place. Where the turns of the melting coil are brought together byconnecting pieces, however, the webs are melted. Thus openings 6 open upa meandering path between the regions which are separated by webs, andthe melting granules 11 have to overcome this path in order to reach themelt 8 on the growing single crystal 9. If the plate is rotated slowly,a web melts as soon as it passes into the region of influence of aconnecting piece. At the same time, the melt is built up again atlocations where molten material leaves the region of influence of theconnecting piece. In this case, the molten material which is situated onthe plate between the separated turns of the melting coil bulges upwardon account of the relatively weak electromagnetic force active there,and ultimately solidifies again.

[0028] A suitably shaped melting coil is illustrated in FIG. 5. It has aplurality of concentric turns 22, the distances between the turns on theinner side being greater than the distances between the turns on theouter side. The turns are connected to one another by connecting pieces23. The hatched areas between the turns which lie further apart indicatethe presence of webs 21.

[0029] The use of an apparatus shown in FIG. 4 is particularlypreferred, since any contact between molten material and quartz parts iscompletely avoided and it is possible to produce a long, meandering pathwhich reliably prevents granules which have not yet completely meltedfrom being introduced into the melt on the growing single crystal. Ifthe melting coil is divided into a coil for the outer region, into whichgranules are introduced, and a coil for forming the meandering path, themelting current can be controlled independently of the melting of thegranules. This is advantageous in particular in the difficult pullingphase of building up the cone.

[0030] A single crystal is produced in a similar manner to the procedurewhich has already been described in connection with the embodiment shownin FIG. 2.

[0031] Silicon single crystals which have been produced using theprocess of the invention make it possible to produce semiconductorwafers with particularly advantageous defect properties. The grown-indefects are smaller than 60 nm even at oxygen concentrations of 3-9*10¹⁷cm⁻³, preferably 4-8.5*10¹⁷ cm⁻³, and particularly preferably 4.5-8*10¹⁷cm⁻³, and are therefore easy to eliminate by heat treatment at least inthe regions where they could adversely affect electronic components.Furthermore, to further reduce the size of the defects and to exciteoxygen precipitation, it is advantageous for the single crystals to beadditionally doped with nitrogen. A nitrogen concentration of1*10¹³-6*10¹⁵, preferably 1*10¹⁴-4*10¹⁵, is expedient.

[0032] Accordingly, while a few embodiments of the present inventionhave been shown and described, it is to be understood that many changesand modifications may be made thereunto without departing from thespirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. A process for producing a single crystal ofsemiconductor material, comprising maintaining fractions of a melt in aliquid state by a pulling coil; solidifying said liquid on a seedcrystal to form a growing single crystal; melting granules ofsemiconductor material in order to maintain growth of the singlecrystal; and passing said melting granules to the melt after a delay. 2.The process as claimed in claim 1, comprising passing the meltinggranules through a system of passages to the melt.
 3. The process asclaimed in claim 2, wherein the melting granules are passed through ameandering system of passages to the melt.
 4. The process as claimed inclaim 1, wherein the melting granules have to overcome at least onebarrier on the way to the melt.
 5. The process as claimed in claim 1,wherein the melting granules travel a distance of at least 25 mm beforereaching the melt.
 6. The process as claimed in claim 1, comprisingcompletely melting the granules; and then feeding said granules to themelt.
 7. The process as claimed in claim 1, comprising effecting themelting of the granules and maintaining of the melt in the liquid stateby an inductive energy supply; and wherein the two operations areinductively decoupled.
 8. The process as claimed in claim 1, comprisingmelting the granules in an outer region of a vessel which is arrangedabove the growing single crystal; and passing said granules to a centralopening in the vessel and, from there, to the melt.
 9. An apparatus forproducing a single crystal, comprising a vessel which is arranged abovea growing single crystal; a conveyor device for feeding granules intothe vessel; a melting coil for melting the granules; a pulling coil formaintaining a melt on the growing single crystal; the melting granulespassing through openings in the vessel and the pulling coil to the melt,so as to form a melt neck, and solidifying fractions of the meltmaintaining growth of the single crystal; and said vessel having adevice which delays mixing of molten granules and of the melt.
 10. Theapparatus as claimed in claim 9, further comprising a protective shieldwhich separates a space around the vessel in a dustproof manner from aspace around the growing single crystal.
 11. The apparatus as claimed inclaim 9, further comprising covers which are arranged above the meltingcoil and which the granules strike when they are fed into the vessel.12. The apparatus as claimed in claim 9, further comprising a device forcooling the vessel.
 13. The apparatus as claimed in claim 12, comprisinga water-cooled metal plate which is separated from the vessel by anarrow gap, thus effecting radiant and convective cooling.
 14. Theapparatus as claimed in claim 9, comprising means for mounting thevessel so that it can rotate about an axis of rotation, said vesselconsists of quartz and has walls made from quartz which divide theinterior of the vessel into concentric regions, these regions being incommunication with one another and forming a system of passages whichthe melting granules have to overcome before they can pass between theopening in the vessel and an ingot of semiconductor material whichprojects through the opening to reach the melt.
 15. The apparatus asclaimed in claim 14, wherein the axis of rotation of the vessel istilted about an angle α.
 16. The apparatus as claimed in claim 9,wherein the vessel comprises a coolable plate of semiconductor materialand has walls made from quartz which divide the interior of the vesselinto concentric regions, these regions being in communication with oneanother and forming a system of passages which the melting granules haveto overcome before they can pass through the opening in the vessel,which is designed as a section of pipe, to the melt, and the apparatuscomprising a radiation heating means for heating a surface of the meltneck and the semiconductor material flowing through the section of pipe.17. The apparatus as claimed in claim 9, wherein the vessel comprises acoolable plate of semiconductor material, which is delimited by an outeredge, and the opening of the vessel is designed as a downwardly directedsection of pipe which is connected to a raised inner edge of the plate,the inner edge of the plate forming a barrier which the molten granuleshave to overcome before they can pass through the opening in the vesselto the melt, and the apparatus comprising a radiation heating means forheating a surface of a melt neck and the semiconductor material flowingthrough the section of pipe.
 18. The apparatus as claimed in claim 9,comprising means for mounting the vessel so that it can rotate about anaxis of rotation, and said vessel comprises a coolable plate ofsemiconductor material and has webs which are constantly remelted underthe influence of the heating coil and the rotation of the vessel anddivide the interior of the vessel into concentric regions, these regionsbeing in communication with one another and forming a system of passageswhich the melting granules have to overcome before they can pass throughthe opening in the vessel, which is designed as a section of pipe, tothe melt, and the apparatus comprising a radiation heating means forheating a surface of the melt neck and the semiconductor materialflowing through the section of pipe.
 19. A silicon single crystalcomprising an oxygen concentration of 3-9*10¹⁷ atoms/cm³ and grown-indefects with a size of less than 60 nm.
 20. The silicon single crystalas claimed in claim 19, further comprising a nitrogen concentration of1*10¹³-6*10¹⁵ atoms/cm³.